The present invention relates to an improved aerator and mixer. The device of the present invention has been designed especially for aeration and mixing of wastewater, and will be described with particular reference to this application. However, it will be appreciated that the device of the present invention could be used in a wide range of other applications where aeration and/or mixing are required.
One known design of aerator/mixer in use at present consists of a rotatable hollow drive shaft with air intake ports at one end, open at the other end, and a propeller adjacent said other end. In use, the aerator/mixer is mounted with the propeller immersed In the wastewater or other liquid to be aerated/mixed, but with the air intake ports above the water line.
The drive shaft is rotated (e.g. by an electric motor) to drive the propeller. The rotation of the propeller mixes the liquid in which the propeller is immersed, and also induces a fluid flow across the lower, immersed, end of the drive shaft. This creates an area of reduced pressure at the lower end of the drive shaft, and hence a similar reduction of pressure at the air intake ports, drawing atmospheric air into the ports and down the shaft.
The air so drawn into the shaft is released as small air bubbles into the liquid flow pattern created by the propeller.
The object of aeration is to introduce oxygen into the wastewater, since oxygen is needed to promote the growth of aerobic micro-organisms which consume the biological nutrients in the waste. The more oxygen is introduced into the wastewater, the greater the rate of growth of the aerobic micro-organisms.
Henry""s Law states that the amount of gas absorbed by a given volume of liquid at a given temperature is directly proportional to the pressure of the gas.
Thus, if the pressure of the air introduced into the wastewater at the lower end of the drive shaft can be increased, the amount of air absorbed by the wastewater will be increased proportionately.
It is therefore an object of the present invention to provide an aerator/mixer which is capable of supplying air to the propeller at above atmospheric pressure, without in any way decreasing the efficiency of the mixing function of the propeller.
The present invention provides an aerator/mixer which includes: a rotatable hollow drive shaft with at least one air intake port at or adjacent one end thereof and a propeller mounted adjacent the other end thereof so as to be rotatable with the drive shaft; means for rotating the drive shaft and the propeller; means for supplying air at above atmospheric pressure to the or each said air intake port an air permeable diffuser mounted on said other end of the drive shaft, with the interior of the diffuser in communication with the interior of said hollow drive shaft such that air supplied to the or each said air Intake port can eat the other end of the drive shaft only through said diffuser; the propeller being further from said other end of the drive shaft than said diffuser.
Preferably, the drive shaft is formed with a plurality of spaced air intake ports.
The means for supplying air at above atmospheric pressure may be any suitable means, e.g. a fan, compressor or blower.
As used herein, the term xe2x80x9cair permeablexe2x80x9d includes any structure through which air can pass, e.g. a perforated plate or a porous material.
In a preferred embodiment of the invention the diffuser comprises a perforated plate. However, for some applications, it may be preferable to form the diffuser from a porous material.
The porous diffuser may be any suitable type of material capable of delivering bubbles of air of a predetermined size (e.g. stainless steel, ceramics). Porous stainless steel providing tortuous paths typically 20 micron average diameter has been found to be suitable for delivering air bubbles of an average diameter of 1.5 mm.